1. Field of the Invention
This invention relates to magnetic recording mediums which have an improved structure of a magnetic layer, thereby attaining significantly improved durability and electromagnetic characteristics. The magnetic recording mediums may be magnetic tapes, floppy disks, magnetic cards and the like.
2. Description of the Prior Art
A recent trend in the field of magnetic recording is to record information at a high density. This requires the use of recording signals in a short range of wavelength. For instance, with VHS (video home system) video tape recorders, a minimum wavelength used is only 1.2 micrometers and for videotape recorders using an 8 mm wide tape, a very short wavelength of 0.6 micrometers has been used. In order to cope with these types of video tape recorders, it is usual to use a coating tape such as a tape comprising cobalt-containing iron oxide particles with a surface roughness of about 500 angstroms as a magnetic recording layer, or a metallic tape having a surface roughness of 200 to 300 angstroms. These known tapes employ a longitudinal recording format in which the magnetization resides in the plane of the tape. However, these types of mediums have a limitation on high density recording as is known in the art.
Instead of the longitudinal recording systems, there have been proposed so-called vertical recording systems which enable one to record information using signals of shorter wavelengths. A typical vertical recording medium is one which comprises a base of a resin such as polyamide, polyimide or polyethylene terephthalate, and an alloy film such as a Co-Cr alloy formed on the base by vacuum deposition or sputtering. The vertical recording medium has significantly improved electromagnetic characteristics including an output level, over known longitudinal or in-plane recording mediums when information is recorded using signals having a wavelength of from 0.1 to 0.5 micrometers.
However, the alloy film formed by vacuum evaporation or sputtering is so thin that it is not highly resistant to abrasion or impact force and is inconveniently liable to break upon contact with a magnetic head or guide rolls. To avoid this, a protective or lubricating layer made of fluorine-containing lubricants or high molecular weight fatty acids is formed on the alloy film so as to improve the durability.
Assuming that information is recorded using a relative speed between a magnetic head and a recording medium of 3.75 m/second and a signal of 7.5 MHz, the wavelength is 0.5 micrometers. When a thickness of the lubricating layer is 100 angstroms, the loss of the reproduction signal caused by the spacing between the head and the medium will result in about 1 dB. This eventually places a limit on the thickness of the lubricating layer. When a magnetic head slidingly moves in contact with such a thin lubricating layer many times, the layer is gradually removed by the contact and cannot thus stand repeated use. In other words, the fine defects originally involved in the alloy film will be enlarged and the alloy film will be partially scraped off. As a result, the alloy dust will deposit on the head surface, causing the head to be undesirably covered with the dust. In an extreme case, the head may be broken by contact with the defective alloy film. Even though the alloy film is not broken, it will often be locally creased based on the defects which are produced by the sliding contact between the head and the tape. This brings about a poor contact between the head and the medium, resulting in a considerable lowering of a signal output level.
In order to overcome the above disadvantage, U.S. Pat. Nos. 4,410,565 and 4,414,271 proposed vertical magnetic recording mediums in which a resin is filled in between grains of a ferromagnetic metallic thin film having a cylindrical grain structure. Alternatively, U.S. Pat. Nos. 4,447,467 and 4,672,009 describe vertical magnetic recording mediums in which a magnetic recording layer is not formed by vacuum evaporation or sputtering but is formed by conventional coating techniques, and is subjected to orientation in a magnetic field. However, these known mediums have not necessarily satisfactory durability. Especially, the mediums obtained by the coating techniques are not satisfactory with respect to electromagnetic conversion characteristics.
In general, a vertical magnetic recording medium having a thin alloy recording film exhibits an output level higher by not less than 10 dB than coated metal tapes in a wavelength region for recording of 0.1 to 0.5 micrometers. On the contrary, however, over 1 micrometer, the output level becomes lower than the level of the coated metal tape, with an attendant problem that a sufficiently high signal-to-noise ratio is difficult to obtain.